The invention relates to a tire with radial carcass reinforcement anchored in each bead to at least one bead wire, and comprising a crown reinforcement formed by at least two so-called working plies, superposed and made of wires or cables which are parallel to each other in each ply and are crossed from one ply to the other, forming angles of at most 45xc2x0 in absolute value with the circumferential direction of the tire.
U.S. Pat. No. 5,738,740 relates a tire as described above, and more particularly a xe2x80x9cheavy vehiclexe2x80x9d type tire, the ratio of the height H above rim to the maximum axial width S of which is at most 0.60. Said application, in order to improve the life of the crown reinforcement of such a tire, and also the regularity of wear of the tread thereof, advocates an architecture of the crown reinforcement characterized by the combined presence in said reinforcement of an axially continuous ply of inextensible cables forming an angle of at least 60xc2x0 with the circumferential direction of the tire, and a ply of metallic elements oriented substantially parallel to the circumferential direction, arranged radially between the two working crown plies.
The object of such an architecture is to reduce the operating temperatures at the edges of the working plies, the width of the additional ply of circumferential cables being less than the widths of the working plies.
Some current tires, called xe2x80x9chighwayxe2x80x9d tires, are intended to travel at high speed and on longer and longer journeys, owing to the improvement in the road network and the growth of the motorway network throughout the world. All the conditions under which such a tire is required to travel without doubt make it possible to increase the number of kilometers traveled, the wear of the tire being less, but on the other hand the endurance of the latter and in particular of the crown reinforcement is adversely affected. The lack of endurance relates both to the fatigue resistance of the crown plies, and in particular the resistance to separation between ends of plies, and to the fatigue resistance of the cables of the portion of carcass reinforcement located beneath the crown reinforcement, the first deficiency being greatly influenced by the operating temperature at the edges of the working plies, whether traveling in a straight line or under drift.
The problems referred to above have been satisfactorily solved by specifying the methods of application of the principle of architecture described above. Thus French application FR 2 744 955 describes a tire with radial carcass reinforcement, of an H/S form ratio of at least 0.60, and having a crown reinforcement comprising at least two working crown plies of inextensible cables, crossed from one ply to the other, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, and, in the absence of any ply formed of inextensible cables forming an angle greater than 45xc2x0 with the circumferential direction, an additional, axially continuous, ply formed of metallic elements oriented substantially parallel to the circumferential direction, placed radially between the working plies, and the axial width of which is at least 1.05 times the axial width of the widest working crown ply, and preferably at least 1.1 times the width of said widest working crown ply.
Although the problems relating to the separation between working plies and the fatigue resistance of the carcass reinforcement cables would appear to be solved if the operating temperatures are greatly reduced, whatever the form ratio of the tire, on the other hand prolonged travel under drift of the tires thus constructed and having a form ratio of at most 0.60 causes fatigue failure of the cables of the additional ply and, more particularly, of the edges of said ply, whether or not the so-called triangulation ply is present.
The object of the invention is to obtain, for the tire in question, the best possible compromise between the different qualities which the reinforcements of such a tire must possess, namely, the fatigue resistance of all the reinforcement elements of the reinforcement plies, the resistance to separation between plies, an operating temperature as low as possible, and the lowest possible tire weight and industrial cost price.
The tire according to the invention, having a radial carcass reinforcement, a crown reinforcement comprising at least two working crown plies of inextensible reinforcement elements, crossed from one ply to the other ply, forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, and an additional, axially continuous, ply formed of metallic reinforcement elements which is placed radially between the working plies, and the axial width of which is at least 1.1 times the axial width of the widest working crown ply, is characterized in that the reinforcement elements of the additional ply are continuous, inextensible, substantially radial metallic elements.
The additional ply advantageously has a substantially zero meridian curvature, and is separated from the first and second working plies by the necessary profiled members, of substantially triangular shape. xe2x80x9cSubstantially zero meridian curvaturexe2x80x9d of said ply is to be understood to mean a curvature, the radius of which is at least 2.00 times the equatorial radius of curvature of said ply.
xe2x80x9cInextensible cablexe2x80x9d is to be understood to mean a cable, for instance a steel cable, which has a relative elongation of less than 0.5%, measured at 10% of its breaking load.
Substantially radial metallic elements are elements which form angles within the range of +85xc2x0, xe2x88x9285xc2x0 around 0xc2x0 with the circumferential direction.
The crown reinforcement according to the invention will advantageously be finished off by a so-called protective crown ply, formed of elastic metal cables made of steel, oriented relative to the circumferential direction at an angle substantially equal to the angle formed by the cables of the radially outermost working crown ply, and the axial width of which is at least equal to the axial width of the radially outermost working ply.
Whatever the solution described above, the crown reinforcement may also be finished off, radially to the inside between the carcass reinforcement and the radially inner working ply closest to said carcass reinforcement, by a so-called triangulation ply of inextensible reinforcement elements, forming with the circumferential direction an angle greater than 60xc2x0 and of the same direction as that of the angle formed by the reinforcement elements of the ply radially closest to the carcass reinforcement. Said triangulation ply may have an axial width less than said widest working ply, which, in the crown reinforcement in question, is radially closest to the carcass reinforcement. Said triangulation ply may also advantageously have an axial width greater than the width of the widest working ply.
The characteristics and advantages of the invention will be better understood with reference to the following description, which refers to the drawings, illustrating in non-limitative manner examples of embodiments of the invention.